Composite materials
by Chris Woodford. Last updated: February 25, 2022.
One plus one equals three is just the kind of math that makes
sense if you're a materials scientist—especially one who works with
composites (the short name for composite materials). Put two useful materials together in a composite and what
you get is a third, somewhat different material that's superior in some crucial way
(maybe stronger or better at handling heat) than either of the
original components: it's more, in other words, than the sum of its
parts.
Composites might sound a little bit "techy" and
unfamiliar, but they're extremely common in the world around us. Bats
for ball sports (tennis rackets, golf clubs, and hockey sticks) are
often made from them. Cars, planes, and boats have long been made
from composites such as fiberglass, because they're lighter than
metals but often just as strong. And if you think composites sound
super-modern, think again: concrete, wood, and bone are all composite
materials. Laminates are composites in which layers of
different materials are bonded together with adhesive, to give added
strength, durability, or some other benefit.
Photo: Testing composite materials onboard
Space Shuttle Mission STS-32, 1990. Picture courtesy NASA JSC Digital Image Collection.
What are composites?
A composite is made by combining two or more other materials so
they improve one another but keep distinct and separate identities in
the final product. So a composite isn't a compound (where atoms or
molecules bind together chemically to make something quite
different), a mixture (where one material is blended into
another), or a solution (where something like salt dissolves in water
and effectively disappears).
A composite is something like concrete, where
stones of various sizes are dotted in between cement. Reinforced concrete is also a composite made
from steel reinforcing
bars placed inside wet concrete—which makes it, in effect, a
composite of a composite. Fiberglass is a composite of tiny glass
shards glued inside plastic. In concrete, reinforced concrete, and
fiberglass, the original ingredients are still easy to spot in the
final material. So in concrete, you can often see the stones in the
cement—they don't disappear or dissolve.
Photo: A simple model of a composite. I've used layers of sticky plastic fastener (Blu-Tack) as the matrix and matchsticks as the fibers, so this is (loosely speaking) a kind of polymer matrix composite. It would be easy to turn this into a science fair experiment: build yourself a large sample of composite like this and then compare its properties to those of the materials from which you've made it.
Why do we need composites?
In at least one important way, a composite must be better than the
materials from which it's made—otherwise there's no point to it.
Considering concrete again, it's very strong if you use it in
vertical beams to take the weight of a building or a structure
pushing down—in other words, if you use it squashed (in
compression). But it's quite weak and tends to bow and then snap if you use
it horizontally, where it's stretched (in tension). That's
obviously going to be a major problem in a building that has lots of
horizontal beams. A great solution is to pour wet concrete around
tight steel bars (called rebars) so that it sets into a composite
material called reinforced concrete. The steel pulls on the
concrete and stops it snapping when it's in tension, while the
concrete protects the steel from rust and decay. What we end up with
is a composite material that works well in both tension and
compression.
Added strength is the most common reason for making a composite, but
it's not the only one. Sometimes, we're looking to make a material better in a different
way. For example, we might need an airplane part with better fatigue
resistance than we'd get from a metal, so it doesn't snap (like a
paperclip) when it's repeatedly stressed and strained in flight. Or
we might need an engine part that can survive at higher temperatures
than an ordinary ceramic. Perhaps we need a plastic that's stiff and
strong but still lightweight, or one that can carry heat and
electricity better than ordinary plastic (something with improved
thermal and electrical conductivity, in other words).
Composites can help us in all these situations.
Photo: The F117 Nighthawk stealth jet planes used clever design and composite materials to evade radar detection. Picture by Lance Cheung courtesy of US Air Force.
How are composites made?
Composites are generally made of two main materials (though there
may be other additives as well): there's a "background" material
called a matrix (or matrix phase) and, to this, we add a transforming
material called the reinforcement (or reinforcing phase).
Although we tend to think of the reinforcement as being made up of fibers (as in fiberglass),
that's not always the case. In reinforced concrete, the "fibers"
are large-scale, twisted steel rods; in fiberglass, they're tiny
whiskers of glass. Sometimes the reinforcement is made of granules,
particulates, or whiskers, but it can also be made of folded
textiles.
The way the particles of reinforcement are arranged in the
matrix determines whether a composite has the same mechanical
properties in every direction (isotropic) or different properties in
different directions (anisotropic). Fibers all pointing the same way
will make a composite anisotropic: it will be stronger in one direction
than another (exactly what we see in wood). On the other hand, particulates,
whiskers, or fibers randomly oriented in a composite will tend to make it
equally strong in all directions.
Whatever form it takes, the reinforcement's job is to withstand forces placed on the material
(adding strength or helping to stop cracks and fatigue), while the
job of the matrix is bind the reinforcement tightly in place (so it
doesn't weaken) and protect it (from heat, water, and other
environmental damage).
Artwork: Anisotropic materials (left) with their fibers pointing the same way will have different properties when stressed form different directions. Isotropic materials (right) with fibers pointing randomly will tend to have the same properties whichever direction they're stressed from.
Types of composites
Natural composites
When we talk about composites, we often mean strong, lightweight,
ultra-modern materials carefully engineered for specific applications
in things like space rockets and jet planes—but
looking at things that way makes it all too easy to forget natural composite materials,
which have been around forever. Wood is a composite made from
cellulose fibers (the reinforcement) growing inside lignin (a matrix
made of organic, carbon-based polymers). Bone is another age-old
composite in which collagen fibers reinforce a matrix of
hydroxyapatite (a crystalline mineral based on calcium). And even
human-made composites aren't necessarily hi-tech and modern. Concrete
and brick (made from mud or clay reinforced with straw) are two
examples of composites invented by humans that have been in
widespread use for thousands of years.
Classic composites
The first modern composite material was fiberglass (originally
spelled "fibreglas" and now generally referred to as glass-fiber
reinforced plastic, GRFP, or GRP), which dates from the 1930s. These days,
GRP typically comes in the form of tapes that can be pasted onto the surface
of a mold. The plastic backing tape is the matrix holds the glass fibers in place, but
it's the fibers that provide much of the material's strength. While
plastic (by definition) is relatively soft and flexible, glass is
strong but brittle. Put the two together and you have a strong,
durable material suitable for things like car or boat bodies, lighter than the metals or
alloys you might otherwise use and not prone to rusting. Carbon-fiber reinforced plastic (CRFP or CRP)
is similar to GRP but uses carbon fibers instead of glass ones.
Photo: Smart cars are lightweight, composite cars.
A steel safety shell holds together a
variety of different parts and panels mostly made of plastics,
including polypropylene (PP), polyvinyl butyral (PVB), polycarbonate (PC),
and polyethylene terephthalate (PET). As on most cars, the "rubber" tires are actually
composites made from rubber and numerous other materials, such as silica.
Modern composites
Today's advanced composites are based on either metal, plastic
(polymer), or ceramic. That gives us the three main types of modern
composite materials: metal matrix composites (MMC), polymer matrix composites
(PMC), and ceramic matrix composites (CMC).
Metal matrix composites (MMC)
These have a matrix made from a lightweight metal such as an
aluminum or magnesium alloy, reinforced with either ceramic or carbon
fibers. Examples include aluminum reinforced with silicon carbide,
and an alloy of copper and
nickel reinforced with graphene (a type of
carbon), which makes the metals several hundred times stronger than
they'd be on their own. MMCs are strong, stiff, hard-wearing,
rust-resistant, and relatively light, but they tend to be expensive
and harder to work. They're popular in aerospace (in things like
jet engines), military applications (steel-boron nitride is used to
reinforce tanks), the automobile industry (diesel engine pistons),
and cutting tools.
Ceramic matrix composites (CMC)
As their name suggests, these use a ceramic material (such as
borosilicate glass) as the background matrix, with carbon or ceramic
fibers (such as silicon carbide) adding reinforcement and helping to
overcome the key weakness of ordinary ceramics (their brittleness and
what's called low "fracture toughness"). Examples include
carbon-fiber-reinforced silicon carbide (C/SiC) and silicon
carbide-reinforced silicon carbide (SiC/SiC). Originally developed
for aerospace and military applications where lightness and
high-temperature performance are really important (such as
gas-turbine, jet engine exhaust nozzles), CMCs have also found uses
in things like automobile brakes and clutches, bearings,
heat exchangers, and
nuclear reactors. Since CMCs tend to be used for
high-temperature applications, polymer fibers and conventional
low-melting glass fibers aren't generally used as reinforcements.
Polymer matrix composites (PMC)
Polymer matrix composites (PMC), such as GRP, are different again.
While the fibers in CMCs make them tougher and less brittle, in PMCs
the ceramic or carbon fibers add strength and stiffness to the
background plastic.
In a PMC, the plastic matrix can be either
a thermoplastic (one that can be softened and reshaped by heating), such as
a polyamide, or a thermosetting plastic ("thermoset"—one that retains its shape after it's made, even
on reheating), such as an epoxy. Generally speaking, PMCs based on thermosetting
plastics are better at surviving high temperatures and attack from solvents
than ones based on thermoplastics, but they're not as tough; they also take longer to make
(because of the "curing" time required) and are less suited to quick, cheap, mass production. As we've just seen, lightness, stiffness, strength, and corrosion resistance make PMCs based on thermosetting plastics, such as fiberglass, excellent materials for car, boat, and plane parts. They're also widely used in sports goods (such
as tennis rackets, golf clubs, snowboards, and skis). Although epoxy-based (thermoset) PMCs are widely
used in aerospace, thermoplastic-based PMCs capable of surviving
high temperatures are becoming increasingly important in these sorts of applications as well.
Photo: An insulating material made of layers (black) of a polymer matrix composite (PMC) separated by aerogel posts (white). So this is another example of a composite that is, itself, made of another composite. Photo courtesy of NASA.
Future composites
A lot of current research is focused on improving composites by
using fibers roughly 1000 times smaller, which pack an awful lot more punch. These so-called
nanocomposites are an example of nanotechnology, using carbon
nanotubes or nanoparticles as the reinforcement. They're likely to
prove both cheaper and to have better mechanical and electrical properties
than traditional composites. Colt Hockey, for example, is now
advertising a carbon-fiber hockey stick coated with nickel-cobalt
nanocomposite that claims to be "2.8 times stronger and 20% more
flexible than steel."
Photo: Nanocomposite: A typical
This brown powder, N-CAS (nanocomposite absorbent solvent), is an
example of a PMC (polymer matrix composite) and it's designed to remove poisonous arsenic
from drinking water. It's made by embedding nanoparticles
of metal oxide, which absorb the arsenic, in a polymer matrix.
Picture courtesy of Idaho National Laboratory and US Department of Energy.
Laminates
Photo: Laminating a paper poster in a heat-treating machine.
Photo by Michael Winter courtesy of US Navy and
Wikimedia Commons.
Having read all about composites, you might have come to the conclusion
that they're not the kind of materials ordinary people are likely to come across
very often—but you'd be wrong! Have you ever fastened sticky-backed plastic onto a book to
protect the cover? Or glued cardboard to paper to make it stronger?
Perhaps you've coated a poster you've printed on your computer with
plastic to make it weatherproof? If you've
done any of these things, you've made yourself a laminate: a
particular kind of composite material formed by bonding together layers of two or more other materials with
adhesives.
What are laminates?
You'll find your dictionary defines a lamina as a thin sheet
or plate of material: a layer, in other words. Fix two or more sheets
of material together and you get a laminate, which is essentially
just a material made up of layers. Since the layers are usually
different materials, laminates are examples of composites, though
the materials aren't integrated together in the same way as
with other (matrix) composites. It's also important to remember that a laminate isn't simply several layers of materials:
the materials have to be permanently bonded together with something like adhesive, so they behave as one
material, not several. You can think of the adhesive (or adhesives—because there might be more than one)
as an additional material in a laminate.
Why would you want to make a laminate? Generally, because a
material you'd normally use by itself (say paper, wood, or
glass)
isn't strong or durable enough to survive by itself. Paper isn't
waterproof, for example, while plastic is relatively hard to print
on. But what if you print on the paper then coat it with plastic? The
laminated composite material you've made gives you the best of both
worlds.
What are laminates used for?
Laminates tend to be based on four main materials: wood, glass, fabric, and paper.
Wood
Laminated floors are very popular because they're really hard
wearing. Unlike a traditional hard wood floor, a laminate floor is
typically made of four layers. The top might be something like a thin
layer of transparent plastic designed to resist stains and scratches.
Underneath that, there's a thin layer of patterned wood (or even paper
printed with a wood pattern) that gives the floor its attractive appearance. The next layer is the core:
the bulk of the material, made from low-grade fiberboard. Finally, on
the base, there's a thin layer of hard, moisture-proof board. Many
low-cost furniture products that resemble solid wood are actually
laminates made of lower-grade wood products (known as chipboard or
particle board) with a thin coating of veneer, plastic, or even
paper. The main drawback of laminated floors is that they can split apart and warp
if they get wet.
Glass
Car windshields and bulletproof glass are
actually very heavy laminates made from several layers of glass and plastic. The outer
layers of glass are weatherproof and scratchproof, while the inner
plastic layers provide strength and a small amount of flexibility to
stop the glass from shattering. You can read more in our main
article about bulletproof glass.
As we've already seen, glass is also laminated with plastic to make composites such as GRP (Glass Reinforced Plastic).
Photo: Bulletproof glass is an energy-absorbing sandwich of glass and plastic. You can think
of it as a composite (because it's a combination of materials) or a laminate (because it involves sheets of
material bonded together). Picture courtesy of US Air Force.
Fabric
Most shoes and many outdoor clothes are made from laminated
materials. A typical raincoat usually has a waterproof membrane
between a hard-wearing outer layer and a soft, comfortable inner layer. Sometimes
the membrane is directly bonded to the inner and outer layers to make
a very tough and durable piece of clothing; this is known as a
3-layer laminate. If the membrane is bonded to the outer fabric with
no inner lining, that's called a 2.5 layer laminate. Waterproof
clothes made this way tend to be more "breathable" than 3-layer
laminates since moisture can escape more easily.
Photo: Looking inside a laminated 2.5-layer waterproof nylon jacket. It looks like a single layer of nylon, but it's actually two layers laminated together. You can tell that because the inner and outer surfaces look totally different. The ultra-waterproof black outer layer is made of rip-stop nylon. The inner white surface is an extra coating that improves air circulation and breathability.
Paper
Many people own small laminating machines that coat pieces of
paper, card, or photographs in a thin but tough layer of durable
plastic. You simply buy a packet of plastic "pouches", insert
your paper item inside, and run this "sandwich" through the
machine. It heats or glues the plastic and presses it firmly together
to make a weatherproof and durable coating. Identification (ID) cards
and credit cards are also laminated with clear plastic so they can survive
several years of use.
